During the exploration of oil and gas, electromagnetic (EM) logging tools are commonly used for the determination of electrical properties of formations. Electrical imagers within wireline tools and Logging While Drilling (LWD) tools have typically both been used to give measurements of the attenuation and phase difference of a formation downhole. However, current imaging tools lack true omni-directional sensitivity and thus fail to accurately represent images in both horizontal and vertical planes of reference.
Aperture antennas may be used as transmitters and receivers in oil based mud and may be used to produce images of a formation downhole. However, images generated using aperture antennas are typically polarized (i.e., have directionality) and cannot provide omni-directional sensitivity. Also, based on design and orientation of imagers using aperture antennas, the signal strength and image resolution of these electrical imagers may be limited.
Another limitation of many electrical imagers is that they cannot operate in oil based muds. Due to the non-conductive nature of some oil based muds, measurements downhole using some electrical imagers may be inhibited. As a result, having the ability to operate in an oil based mud environment is one advantage some electrical imagers have over others. For example, galvanic type electric imagers do not work in oil based muds because they exhibit very low conductivity in such environments. Also, operating electrical imagers with higher frequencies (in the range of hundreds of Megahertz to Gigahertz) may be necessary for imaging in oil based mud.
FIG. 1A illustrates an electrical imager configuration according to the prior art. The imager consists of an array of two transmitters and two receivers sharing a similar size and being oriented in a one dimensional array. These transmitters and receivers are used to transmit high-frequency electrical signals into a formation and receive the reflection of the transmitted signals for imaging. Referring to FIGS. 2A and 2B however, when imaging the phase and attenuation of a formation downhole using the imager as discussed above, the imager fails to accurately represent the resolution of formation fractures in both the horizontal and vertical directions. FIG. 2A illustrates an image of the attenuation of vertical and horizontal fractures downhole having equal widths, whereas FIG. 2B illustrates an image of the phase difference of the vertical and horizontal fractures in the formation.
As illustrated in FIG. 2A, when measuring the attenuation of the formation downhole using the electrical imager configuration above, the fractures in the vertical direction (vertical fracture) appear darker than the image of the fractures in the horizontal direction (horizontal fracture). Because the fractures in the formation have equal widths, the contrast of the vertical fracture and the contrast of the horizontal fracture should be the same. Thus, imagers in the art may not accurately represent the fractures in a foundation adequately. As a result, the data from these imagers may be misleading.
Referring to FIG. 2B, an image of the phase difference of the formation is shown. As shown, the horizontal fracture is shown to be substantially darker than the vertical fracture. As discussed above, because the fractures have the same width, the contrast of the vertical fracture and the horizontal fracture should be the same. In true omni-directional imaging, contrasts representing fractures in a formation are accurately represented.
Also, current imagers lack the ability to provide a quantitative analysis of the resistivity and dielectric constant of a formation downhole. By measuring the resistivity and dielectric constant of a formation, geologists and petrophysicists can more accurately evaluate formation parameters such as permeable layers, water and/or oil reservoirs, fractures, strike angles and/or dipping angles, etc. Current imaging tools are mostly limited to imaging and cannot provide both high resolution imaging and a quantitative analysis of formation resistivity and dielectric constant.
It is therefore desirable to have an electrical imager system and method for creating, an omni-directional, high resolution image of the parameters of a downhole formation such as the phase difference and attenuation in oil based mud, while also being capable of providing a quantitative analysis of formation resistivity and dielectric constant.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.